Off and on for a few months, the range hood fan in my Whirlpool over the stove microwave has tried to come on whenever I've boiled water on the stovetop. I have assumed that the steam is setting off the sensor that turns the fan on automatically. Typically it buzzes a few seconds and stops, buzzing off and on for as long as the water is steaming and for a short while thereafter.

Yesterday it continued to buzz and stop for hours and sometime during the night started buzzing and didn't stop. I got up in the middle of the night and it smelled like the motor was starting to burn up, so I unplugged it.

I plugged it back in this morning and if I turn on the fan, it runs normally. If I turn it off, it buzzes. I pulled off the cover and I can see the fan running slowly when it is turned off.

The microwave lights, heats and cooks normally. It's about 3 years old.

What's wrong and how do I fix it?

Thanks.